The present invention relates to a method and apparatus using an electron beam to measure a displacement between microstructured patterns, and to a scanning electron microscope using a microstructured-pattern displacement measuring apparatus.
With a growing tendency towards finer LSI circuit pattern structuring, reduction in the magnitude of an overlay error, in addition to the management of pattern sizes, is becoming a vital issue. The overlay error here is an error in the distance between the patterns formed in different processes, with respect to a design value. The layout of patterns is designed so that a silicon (Si) substrate on which a groove pattern has been formed in a process is coated with a resist in another process and then subjected to lithography to form a plurality of line patterns at equidistant positions from the groove pattern. During actual patterning, however, the center of the space region defined by the line patterns usually does not align with the center of the groove pattern. This amount of misalignment between the central position of the line patterns space region and that of the groove pattern is called the overlay (OL) error.
From time to time, each of the three patterns is originally designed so as not to align, in which case the amount of misalignment between a design value and an actual value is often defined as the OL error. In recent years, patterns that have gone through different forming processes exist on the same layer in some cases. For example, line patterns are formed on a Si substrate by the execution of first lithography and then other line patterns are formed on the same substrate by the execution of second lithography. The OL error between the line patterns that have been formed during the second lithography becomes the difference between the center of the line patterns that have been formed during the first lithography, and that of the line patterns formed during the second lithography.
In many cases, OL errors have heretofore been optically measured with the edge positions of patterns as its basis, as disclosed in Japanese Patent Applications JP-2000-88702A and JP-2009-270988A.
However, the tool-induced shift TIS, caused as a measurement error due to the measuring device during the optical measurement of the OL error, has become a problem. The tool-induced shift TIS is an error due to the angle of incidence of irradiation light or to the asymmetry of the irradiation light.
One of the technical solutions considered in JP-2000-88702A is by, after the measurement of the OL error, rotating the target object through 180 degrees with respect to the irradiation light and then canceling out the OL error with the data obtained by the rotation. The correction of other problems such as aberration, as well as the simple correction of the angle of incidence, is also reviewed.
The idea of correcting the TIS itself can be a further solution, as in JP-2009-270988A. For optical measurement, since the focal position during observation has a relationship with TIS, a method of optimizing this relationship has been disclosed.
However, needs of users who wish to measure the OL error as near as possible to vital patterns in the chip are arising in recent years. This is because, although measurements have formerly been conducted with an OL error measurement target pattern disposed at several locations including the corners of the chip, this has been insufficient to accurately predict the OL error of the vital patterns and a yield has not been improvable. Despite those needs, the optical measurement target patterns cannot be disposed at the desired positions since these target patterns are as large as tens of micrometers (μm) on one side.